1. Field of the Invention
The present invention relates to a transmission characteristic compensation control scheme, in particular, a transmission characteristic compensation control scheme in which a plurality of compensation circuits are used for compensation of transmission characteristic degradation.
2. Description of the Related Art
In recent years, in improvement in transmission rate in an optical transmission device, an optical transmission device 10 Gb/s has been already put in practical use. As for an optical transmission device of 40 Gb/s, development has also been preceded. Thus, in an optical transmission device, degradation in optical signal waveform due to wavelength dispersion in optical fiber, polarization dispersion in optical fiber or optical circuit, non-linear effect, amplified spontaneous emission (ASE) or the like may become remarkably problematic and thus, may limit permissible transmission distance thereof.
Accordingly, in case transmission is made for hundreds of km or more by an optical transmission device of 40 Gb/s, a plurality of transmission characteristic compensation control systems for compensation of degradation in optical signal waveform occurring due to various factors mentioned above are required.
For example, an example of a transmission characteristic compensation control system which compensates transmission characteristic degradation using a plurality of compensation circuits is disclosed in Japanese Laid-Open Patent Application No. 9-326755. Therein, a plurality of compensation circuits, such as, for example, a variable dispersion equalizing circuit, and so forth, and a control circuit which controls the compensation circuits are separately provided, and, the control circuit performs control of the plurality of compensation circuits in a centralized manner.
However, in such a transmission characteristic compensation control scheme, the control circuit performs centralized control of the plurality of compensation circuits, and thus, in case the number of compensation circuits increases, control by the control circuit should become complicated.
Therefore, this type of transmission characteristic compensation control scheme may have a problem that it is difficult to increase the number of compensation circuits according to a rise in transmission rate, extension of transmission distance, etc. Thus, it may not be sufficiently flexible or may not have a sufficient adaptability with regard to system extension and system modification.
For such a transmission characteristic compensation control scheme, a so-called “simple method” and a “two-dimensional method” have been proposed for parameter searching method in compensation circuits.
The simple method is a method by which, in case there are two types of control objects, parameters are set for providing a minimum code error rate on the first control object, and, then, the parameters are set again for providing a minimum code error rate on the second control object. However, as the two parameter are not individual in general, it was not possible to provide a sufficient accuracy on code error compensation by the simple method.
The two-dimensional method is a method in which, code error measurement is made on all the combinations of the two types of control objects, and, then, the optimum parameters are obtained from the measurement result. However, the two-dimensional method needs to increase the number of combinations for improving the accuracy on searching the optimum parameters concerning the compensation operation, and, improvement in the accuracy should result in increase in time required for control of the compensation circuits. Moreover, in control of the compensation circuits at a time of actual operation, circuit switching etc. may take place as error increases. This may make the compensation control more complicated. Accordingly, it may not be realistic to perform code error measurement on all the combinations of control objects according to the two-dimensional method.